The short answer: between 56,135 and 140,750 vehicles a day, depending entirely on where you join it. The M3 is not one road with one level of traffic — it is a quiet stretch across Hampshire bolted to one of the busiest pieces of motorway in the south of England.

The M3 junction by junction, in order

Every mainline stretch the Department for Transport measures, ordered from busiest to quietest, with the traffic broken down by vehicle type. 2025 figures.

Where the M3 changes character

Three places on the M3 change the traffic completely:

• Junction 2, the M25. Around 45% of the traffic swaps motorway here. North of it the M3 carries 62,848 a day; south of it, 113,905.
• Junction 8, the A303. Roughly half the traffic leaves for the West Country, dropping the M3 to its quietest measured stretch.
• Junction 9, the A34. It almost doubles again — 56,135 becomes 110,741 in a single junction, and the lorry share jumps from 6.3% to 9.4%.

From Junction 9 south it never really recovers: 110,741, then 126,217, 128,023, 130,785 and finally 140,750 into the M27 — about 98 vehicles every minute, averaged day and night across the whole year.